Chromatography is the separation of mixtures of chemical substances into their component parts by chromatographic adsorption for analytical purposes. Chromatographic adsorption is the preferential adsorption or differential retention of chemical compounds on the basis of molecular size, charge, hydrophobicity, or biospecific affinity by an adsorbent material. Liquid chromatography is a form of chromatography which employs a liquid as the "mobile phase" and a solid, or a liquid on a solid support, as the stationary adsorbent phase. A liquid chromatography system typically includes the following elements connected together in a manner well known to those of ordinary skill in the field: a column, a pump, an injector, a reservoir, a detector, and a recorder.
In column chromatography, the liquid mobile phase is introduced into the top of a cylindrical column and forced through a bed of material to separate the liquid into various components. Solvents and chemical reagents are used to facilitate the separation. Therefore, the chromatography column, associated valves, and any other related apparatus should be made of materials which will not react with solvents or chemical reagents. For biological separations, sterilizable materials may be necessary. Glass is frequently chosen as the material for the column because it is resistant to common solvents, reagents, sterilizing agents and heat, and its transparency permits an analyst to watch the separation as it takes place.
Pressure chromatography employs a positive pressure within the head of the column, usually about three psi (about 2 N/cm.sup.2), to force the mobile phase through the bed of material. The pressure head above the liquid has commonly been controlled by a pressure regulator valve outside the column. However, the pressure which glassware can withstand is minimal. If the pressure within the column significantly exceeds the normal operating pressure, a glass column may shatter, thus ruining the separation and creating a hazard. Despite this problem, pressure relief valves have not been used in pressure chromatography columns.
At the top of a pressure chromatography column is a mouth, through which the bed is introduced. Before or after the mobile phase is introduced, the mouth is capped so a pressure above ambient pressure can be maintained within the column. Positive pressure is maintained by supplying a low-pressure inert gas, such as helium, to the column through its cap. A reservoir may also be connected to the head of the column to increase the amount of mobile phase which may be contained in the column. In that case, the reservoir has a mouth which is capped, and an inert gas is introduced to the reservoir to pressurize the reservoir and the column.
The mouth of the column or reservoir, and thus its cap, frequently has a smaller diameter. The cap must have two, three or even four independent apertures for admitting inert gas, the mobile phase containing the sample to be analyzed, a fresh elution solvent, a thermometer, or other materials or apparatus without releasing the pressure within the column. Consequently, the cap has little space in which additional apparatus can be mounted.
In another refinement of general liquid chromatography, specific modifications in the design and nature of the column, the stationary adsorbent phase contained in the column, the injector, and the chromatographic conditions (e.g., pressure, temperature, flow rate, solvent properties) are made in order to provide improved separation and resolution for more refined analysis of particular samples. This refinement of liquid chromatography, well understood by those of ordinary skill in the field, is termed high performance liquid chromatography ("HPLC"), and has achieved widespread application in the analysis of a multitude of biological and chemical samples.
The typical mobile phase reservoir in an HPLC system is a glass container which may have been designed specifically for use in such a system. In a manner similar to that described above with regard to pressure chromatography, pressure is used in HPLC reservoirs for two reasons: (i) to keep atmospheric gases away from the liquid mobile phase contained in the reservoir; and (ii) to supply the mobile phase to the system's pump at a slight positive pressure, thus improving the performance of the pump's check valves. Furthermore, an HPLC reservoir can be accidentally pressurized during the widely used technique of helium sparging. This technique consists of bubbling helium gas through the mobile phase contained in a glass reservoir. If no outlet vent for the helium is provided, the glass reservoir will become pressurized, and could explode.
As discussed above in relation to pressure chromatography, the mouth of the mobile phase reservoir typically has a relatively small diameter, and therefore a cap for the reservoir has a small diameter as well. Moreover, and again as discussed in relation to pressure chromatography, the cap should contain at least the following independent apertures: an aperture for helium sparging; for egress of the mobile phase from the reservoir; and for egress of the helium used for sparging. The cap may also contain simple, manually opened and closed "on/off" valves for controlling sparging; for mobile phase egress to the pump, and for venting or sealing during sparging or blanketing, respectively. The cap may also possess filtration capabilities. Therefore, space considerations may be even more critical in a cap for a mobile phase reservoir in an HPLC system than in a pressure chromatography system.
Valves are commonly used in pressure vessels to control the flow and pressure of fluids. Most common valves include a body or housing, a seat, a disk or sealing element, and means for urging the sealing element into sealing engagement with the seat to restrict flow or maintain pressure into or out of the vessel.
The inventors have found that Kalrez fluorocarbon material ("Kalrez" is a trademark of E.I. du Pont de Nemours & Co., Wilmington, Del.) is useful as a seat material in a valve for pressure chromatography. However, Kalrez is not readily available in durometers of less than 75 Shore A. Such a high durometer seat material requires that the sealing element of a valve be urged into the seat with a substantial seating force (expressed as pounds per square inch or newtons per square centimeter of valve disk to seat contact area) to deform the seat material and create a positive seal. F
In valves which may have a large cross-section, or which maintain a relatively large pressure differential, the area of the seat contacted by the valve disk may not be critical. In a large valve, the effective piston area of the sealing element can be large enough that a small head of pressure controlled by the valve can counteract a significant seating force and force the valve open. (The "effective piston area" of a valve disk is defined here as the area of the seated valve element normal to its direction of unseating upon which the pressure at the valve inlet acts and thus urges the element toward an unseated position.) In high pressure applications, the substantial pressure acting on even a small effective piston area of the sealing element can be sufficient to counteract a significant seating force.
It is difficult to design a low pressure differential, small cross-section pressure relief valve which exerts a high enough seating force per square unit of cross-sectional area of the sealing surface to deform the seat while having a substantial enough effective piston area to develop a force sufficient to counteract the seating force when pressure relief is desired.
Yet another problem in pressure relief valve design is how to design a valve which has a few parts and is easily assembled and disassembled for repair or cleaning.
In addition, and particularly in HPLC systems, loose fitting HPLC mobile phase reservoir caps are unable to maintain positive pressure in the mobile phase reservoir. Caps unable to maintain positive reservoir pressure require continuous helium sparging to maintain the mobile phase in a degassed state. Continuous sparging wastes helium and can lead to venting of environmentally undesirable organic solvents. Caps unable to maintain positive reservoir pressure can also cause the check valves in the HPLC system's pump to operate in an unreliable manner. However, caps which provide a tight fit are unable to avoid over-pressurization within the glass reservoir and can create a hazardous situation in the laboratory.